1. Field of the Invention
The present invention relates to a developing device used in an electrophotographic field, wherein an electrostatic latent image is visually developed by using a one-component developer, particularly a non-magnetic type one-component developer.
2) Description of the Related Art
As is well known, an electrophotographic printer carriers out the processes of: producing a uniform distribution of electrical charges on a surface of an electrostatic latent image carrying body such as an electrophotographic photoreceptor; forming an electrostatic latent image on the electrically charged surface of the electrophotographic photoreceptor by optically writing an image thereon, using a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e., toner, which is electrically charged to be electrostatically adhered to the electrostatic latent image zone; electrostatically transferring the developed visible image to a paper; and fixing the transferred image on the paper. Typically, the electrophotographic photoreceptor is formed as a photosensitive drum having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof.
In the developing process, a two-component developer composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers) is widely used, as it enables a stable development of the latent image. Note, typically the toner particles have an average diameter of about 10 .mu.m, and the magnetic fine carriers have a diameter ten times larger than the average diameter of the toner particles. Usually, a developing device using the two-component developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to tribeoelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes magnetic roller, provided in the vessel as a developing roller, in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon.
In this developing device, a ratio between the toner and magnetic components of the developer body held in the vessel must fall within a predetermined range, to continuously maintain a stable development process. Accordingly, the developing device is provided with a toner supplier from which a toner component is supplied to the two-component developer held in the vessel, to supplement the toner component as it is consumed during the development process, whereby the component ratio of the two-component developer held by the vessel is kept within the predetermined range. This use of a two-component developer is advantageous in that a stable development process is obtained thereby, but the developing device per se has the disadvantages of a cumbersome control of a suitable component ratio of the two-component developer, and an inability to reduce the size of the developing device due to the need to incorporate the toner supplier therein.
A one-component developer is also known in this field, and a developing device using same does not suffer from the above-mentioned disadvantages of the developing device using the two-component developer, because the one-component developer is composed of only a toner component (colored fine synthetic resin particles). Two types of the one-component developer are known; a magnetic type and a non-magnetic type. A developing device using the magnetic type one-component developer can be constructed in substantially the same manner as that using the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by a rotating magnetic roller as in the developing device using the two-component developer. The magnetic type one-component developer is suitable for achromatic color (black) printing, but is not suitable for chromatic color printing. This is because each of the toner particles composing the magnetic type one-component developer includes fine magnetic powders having a dark color. In particular, the chromatic color printing obtained from the magnetic type one-component developer appears dark and dull, due to the fine magnetic powders included therein. Conversely, the non-magnetic type one-component developer is particularly suitable for chromatic color printing because it does not include a substance having a dark color, but the non-magnetic type one-component developer cannot be brought to the surface of the photosensitive drum by the magnetic roller as mentioned above.
A developing device using the non-magnetic type one-component developer is also known, as disclosed in U.S. Pat. Nos. 3,152,012 and No. 3,754,963, Japanese Examined Patent Publication (Kokoku) No. 60-12627, and Japanese Unexamined Patent Publications (Kokai) No. 62-976, No. 62-118372, No. 63-100482, and No. 63-189876. These developing devices include a vessel for holding the non-magnetic type and-component developer, and a conductive elastic roller provided within the vessel as a developing roller in such a manner that a portion of the elastic roller is exposed therefrom and can be pressed against the surface of the photosensitive drum. The conductive elastic developing roller may be formed of a conductive silicone rubber material or a conductive polyurethane rubber material or the like. When the conductive rubber roller is rotated within the body of the non-magnetic type one-component developer held by the vessel, the toner particles composing the non-magnetic type one-component developer are frictionally entrained by the surface of the conductive rubber developing roller to form a developer layer therearound, whereby the toner particles can be brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. In this developing device, the development process is carried out in such a manner that, at the area of contact between the photosensitive drum and the conductive rubber developing roller carrying the developer layer, the charged toner particles are electrostatically attracted and adhered to the latent image due to a bias voltage supplied to the conductive solid rubber developing roller.
The developing device further includes a blade member which is resiliently pressed against the surface of the developing roller, to uniformly regulate a thickness of the developer layer formed therearound so that an even development of the latent image can be carried out. The blade member may be also used to electrically charge the toner particles by a triboelectrification therebetween and/or by a charge-injection effect resulting from supply of voltage to the conductive blade member. Of course, when the charge-injection effect is utilized, the blade member is formed of a conductor such as a conductive rubber material, aluminum, stainless steel, brass or the like.
The supply of voltage to the blade member also serves to prevent an electrostatical adhesive of the toner particles to the blade member during the regulation of a thickness of the developer layer formed around the developing roller.
The developing device may be also provided with a toner-developing roller which is rotated in contact with the developing roller so that remaining toner particles not used for the development of the latent image are mechanically removed from the developing roller, whereby a fresh developer layer can be continuously formed on the surface of the developing roller. Note, the developer layer formed of the remaining toner particles not used for the development of the latent image should be once removed from the developing roller because it is subjected to physical and electrical affects during the developing process, and thus prevents a physical and electrical even formation of the developer layer around the developing roller. The toner-removing roller is frequently formed of a conductive porous rubber material, so that the remaining toner particles not used during the development of the latent image can be also electrostatically removed from the developing roller. In particular, a voltage is supplied to the conductive toner-removing roller so that the charge toner particles are electrostatically attracted from the developing roller to the toner-removing roller, whereby not only the mechanical removal of the remaining toner particles but also the electrostatic removal thereof can be carried out.
Conventionally, when a drive motor is driven for the rotation of the developing roller, the voltages are simultaneously supplied to the developing roller, the blade member, and the toner-removing roller, respectively. Nevertheless, the rotation of the developing roller cannot be started at exactly the same time as voltages are supplied to the developing roller, the blade member, and the toner-removing roller, due to a backlash of a gear train provided between the drive motor and the developing roller. Namely, before the rotation of the developing roller is started, the voltages are supplied to the developing roller, the blade member, and the toner-removing roller, and thus a high electrical current flows at localized contacting zones between the developing roller and the blade member and toner-removing roller, resulting in a fusion of the developing roller at the localized contacting zones due to the generation of Joule heat. Of course, when the surface of the developing roller is damaged, the quality of a developed image( and thus a printed image) is deteriorated.
Also, conventionally, when the developing device is in a non-operative condition, the developer layer formed around the developing roller is left as it is. Accordingly, the toner particles existing between the photosensitive drum and the developing roller may be adhered not only to each other but also to the surface of the photosensitive drum, because the developing roller is resiliently pressed against the photosensitive drum, and thus when the developing device is again operated the quality of a printed image is deteriorated.